Processes are known in the lighting industry for applying a thin protective coating to the outer surface of individual phosphor particles in order to improve the performance properties, e.g., lamp maintenance, of the phosphor when used in lamps.
One such known process applies such coating to the individual phosphor particles employing a fluidized bed which is maintained in a temperature gradient. See U.S. Pat. No. 4,585,673 "Method For Coating Phosphor Particles" of A. G. Sigai, issued 29 Apr. 1986, the disclosure of which is hereby incorporated herein by reference.
Such coating processes can experience operational problems if the phosphor powder becomes too fluid in the bed. Very fluid or mobile phosphor powders transfer heat efficiently from the hot upper zone of the reactor to the lower zone. One possible consequence of such high powder mobility is a loss of the temperature gradient in the system leading to unacceptably high temperatures in the lower section of the reactor. This often may lead to undesired pyrolysis of precursors at the distributor plate causing plugging of the plate and a loss in conversion efficiency. In addition, the high temperatures at the reactor bottom often may lead to failure of gaskets, which is undesirable when using flammable or pyrophoric chemicals.